In Standard Specification 3GPP TS24.301, it is specified that a Mobility Management Entity (MME) that manages a User Equipment (UE) may include a New Tracking Area Identity List (NEW TAI List) allocated to the UE and the Globally Unique Temporary Identity (GUTI) of the UE in a GUTI Reallocation Command message and deliver the message to the UE. If the MME detects a Lower Layer Failure before it receives a GUTI Reallocation Complete message fed back by the UE, then in order to establish a Non Access Stratum (NAS) signaling connection between the UE and the network side, the MME may page the UE by using the International Mobile Subscriber Identity (IMSI) of the UE.
Additionally, during Tracking Area Update (TAU) procedure, the MME may include the NEW TAI List allocated to the UE and the GUTI of the UE in a Tracking Area Update Accept (TAU Accept) message and deliver the message to the UE. If the MME detects a Lower Layer Failure before receiving a TAU Complete message fed back by the UE, then the MME pages the UE by using the OLD GUTI or NEW GUTI of the UE; if results of the paging with the OLD GUTI and the NEW GUTI both present a paging failure, then the MME pages the UE by using the IMSI of the UE, and the UE may access to the network again after receiving the paging from the MME.
A mobility management context of the UE containing a TAI LIST of the UE is stored in the MME. During the procedure of paging the UE, the MME determines Tracking Areas (TAs) in which the UE is registered from the TAI LIST in the mobility management context, and then sends a paging message in those TAs in which the UE is registered. During the above GUTI Reallocation procedure and TAU procedure, if no NEW TAI List is allocated to the UE by the MME, then the existing TAI List of the UE stored in the MME is effective, that is, the OLD TAI List is effective, and paging the UE by the MME using the IMSI of the UE is in those TAs corresponding to those Tracking Area Identities (TAIes) contained in the OLD TAI List; or if a NEW TAI List is allocated to the UE by the MME, and the MME is unable to determine the effectiveness of the NEW TAI List and the OLD TAI List after detecting a Lower Layer Failure, that is, the MME cannot determine whether the NEW TAI List allocated to the UE is effective or the existing OLD TAI List of the UE is effective, then the MME cannot determine TAs in which the MME may page the UE by using the IMSI of the UE.
Additionally, if the MME loses the mobility management context of the UE due to such reasons as restart, or if the mobility management context of the UE is corrupted, it may cause the MME unable to determine the TAI LIST of the UE. Thus, the MME cannot determine the TAs in which the MME may page the UE by using the IMSI of the UE, either.
When a network error, for example, Lower Layer Failure or MME Restart, is detected, the UE may be unable to access to the network, and at this point, it needs to page the UE in order to make the UE to access the network again. In conclusion, during the above network error recovery procedure, a situation in which the MME cannot determine the TAI List of the UE when the MME pages the UE by using the IMSI of the UE may exist, and at this point, the MME cannot determine TAs in which the MME pages the UE.